Isomerization and dissociation processes of protonated benzene and protonated fulvene in the gas phase

The protonation energetics of benzene and fulvene are investigated through the use of molecular orbital ab initio G2(MP2) calculations using B3LYP/6-31G* optimized geometries. Calculated proton affinities of benzene and fulvene are 752 and 842 kJ mol(-1) assuming formation of benzenium ion 1 and C alpha-protonated fulvene 3, respectively. Isomerization reactions of protonated benzene and protonated fulvene were investigated at the same level of theory. The isomerization of benzenium ion 1 into protonated fulvene implies as a first step the formation of bicyclo[3,1,0]-hexenyl cation, 5, that may eventually evolve to yield methylene-protonated fulvene 2 and C alpha-protonated fulvene 3. The latter yields the C-beta-protonated species 4 through a relatively low activation barrier. The energy required by the dissociation of benzenium ion via a 1,1-H-2 extrusion process is equal to its endothermicity. The excess energy of the dissociating species 1 having a lifetime of 10(-5) s is sufficiently low to explain the absence of an appreciable kinetic energy release during the separation of the products. (Int J Mass Spectrom 185/186/187 (1999) 241-251) (C) 1999 Elsevier Science B.V.